64 ASTRONOMY t FEBRUARY 2014
- Clusters composed of bright and faint
stars; generally a few very bright and some
moderately bright stars standing out from a
host of fainter ones.
Finally, Trumpler divided clusters based
on the number of stars they contained. He
thought this was the most important dis-
tinction, and designated them by letters:
p. Poor clusters with less than 50 stars.
m. Moderately rich clusters with 50 to
100 stars.
r. Rich clusters containing more than
100 stars.
This system, then, contained 36 types of
clusters ranging from I1r (sometimes you’ll
see I-1-r) to IV3p (or, IV-3-p). He also
allowed for the addition of three capital
letters if the cluster had a pronounced
peculiarity: E for elongated, U for unsym-
metrical, and N for nebulosity involved in
the cluster. The only one you’re likely to
encounter is N, and today you’ll sometimes
see the lowercase n.
So, for example, each of the two stellar
aggregates in the Double Cluster in Perseus
(NGC 869 and NGC 884) carries the Trum-
pler classification IV3r. And the Pleiades
star cluster (M45), with its associated nebu-
losity, has the classification II3rN.
A great astronomer
On September 10, 1956, Trumpler died
after several years of failing health. His
contributions to astronomy brought him
wide recognition. He belonged to many
scientific societies, including the National
Academy of Sciences, which elected him to
membership in 1932. The Astronomical
Society of the Pacific elected him president
in 1932 and 1939 and has established an
award in his memory, given annually to a
promising postdoctoral astronomer.
His legacy also lives on for amateur
astronomers every time one of us points a
telescope at an open cluster and asks not
“What cluster is that?” but rather “What
type of cluster is that?”1r 1m 1p 2r 2m 2p 3r 3m 3p
I NGC 6 819 N G C 179 8 N G C 12 20 NGC 2506 King 16 Ber 4 NGC 4755 N G C 1502 NGC 7160
II NGC 6939 NGC 1883 NGC 1624 NGC 7789 NGC 2192 Cz 6 NGC 6791 NGC 2632 M18
III N G C 6 811 Ber 67 NGC 225 NGC 6940 NGC 1348 NGC 7686 M35 N G C 7423 NGC 6357
IV Rp 101 NGC 2482 NGC 6846 N G C 1817 NGC 956 NGC 6738 M25 NGC 6507 DoDz 6
Key: Ber = Berkeley; Cz = Czernik; DoDz = Dolidze-Dzimselejsvili; NGC = New General Catalogue; Rp = RuprechtTRUMPLER OPEN CLUSTER CLASSIFICATIONS
The first three groups form a series with
decreasing central condensation. The
fourth is less like the others and more like
the background fields of view these objects
f loat in front of. Trumpler added it on the
basis of his results from compiling star
cluster distances.
Next, he subdivided each of the four
main groups into three types according to
the range in the brightness or luminosity of
the cluster stars:- Most cluster stars nearly of the same
apparent brightness. - Medium range in star brightnesses.
SEE A GALLERY OF CLASSIFIED CLUSTERS AT http://www.Astronomy.com/toc.NGC 1817 (Trumpler classification IV2r) lies to the left of open cluster NGC 1807 (Trumpler classifica-
tion II2p). An 8-inch scope will let you count 100 stars in NGC 1817.NGC 7790 (lower left of center) in Cassiopeia shines at magnitude 8.5 and has a Trumpler designation
of II2m. Also shown is NGC 7788 (upper right of center), which has a Trumpler classification of I2p.